Paper products are in frequent use in today's society. Core wound paper products, such as toilet tissue and paper towels, often have a hollow tubular core about which a roll of the product is wound. A consumer usually does not use the entire roll of the paper product at once. To aid the consumer in selecting and dispensing the proper portions of the product, the roll of paper product is conveniently provided with lines of weakness, generally parallel to the axis of the core about which the paper product is wound. The lines of weakness typically comprise perforations which divide the core wound paper product into individual sheets joined across the perforations, yet easily separated from one another. The perforations provide for incremental dispensing of individual and multiple sheets of the product. This feature allows the consumer to conveniently dispense a particular quantity of the product at his or her convenience. The product may also be provided in a stack. The individual sheets of the product are folded in the stack to be on the top of one another, while still divided by perforation lines.
The perforations may be made by perforating surfaces employed during the manufacturing process. During the perforating step of the manufacturing process, the web is typically interposed between two mating perforating surfaces, usually termed as a blade and an anvil, one of which (for example the blade) is attached to the perforating roll, and the other (the anvil) is attached to the stationary backing member. The blades are typically mounted on a rotating roll, or cylinder, and have alternately spaced teeth and notches across the total width of the perforator perforating surface. The teeth are responsible for the small cuts which define and divide adjacent sheets of the web product, while the notches are responsible for the lands which bridge adjacent sheets and hold the roll of sheets together. The blades attached to the rotating perforating roll strike the web while it is held against the stationary anvil and cut through the thickness of the web product.
Different web products require different sheet lengths. Therefore, today's industrial processes often demand that the length of the individual sheets in the perforated web be changed relatively fast and with minimal effort. The change of the length of the individual sheets may be accomplished by substituting one perforating roll for another perforating roll. For example, if it is desired that the length of the individual sheets in the perforated web be increased, one perforating roll may be substituted for another roll having relatively fewer perforating surfaces. The roll having the perforating surfaces spaced relatively farther apart from each other will provide a greater distance between the lines of perforation in the web. The change of the perforating rolls, however, unavoidably creates long periods of downtime and, consequently, is economically undesirable.
Commonly assigned U.S. Pat. No. 4,687,153, issued Aug. 18, 1987 to McNeil, and incorporated by reference herein, teaches that the length of individual sheets in a perforated web can be adjusted by independently controlling the velocity of the web relative to the surface velocity of the perforating roll. This patent provides great advantages over the prior art by allowing to control the length of the individual sheets in the perforated web without the necessity of changing one perforating roll for another. However, flexibility in adjusting the length of the individual sheets in the perforated web without changing the perforating roll is still somewhat limited. A range of the velocity differential between the perforating roll and the web is limited by the web's capability to accept the so called "adhering" effect of the velocity differential. At certain levels of the velocity of the web, the velocity differential between the perforating roll and the web may cause the individual sheets to partially separate along the perforations ("perforation popping") and may even lead to breakout of the web. It has been found, for example, that some paper tissue webs tolerate only up to about -20% ("underspeed") velocity differential between the perforating roll and the web. As used herein, the term "underspeed" indicates that the peripheral velocity of the perforating roll is less than the velocity of the web. By analogy, the term "overspeed" indicates that the peripheral velocity of the perforating roll is greater than the velocity of the web. Overspeed is limited by several considerations, the important one of which is avoiding vibration of the rewinding equipment at the industrial-scale velocities, which vibration was found to take place at the overspeed levels of about +100% and greater. Therefore, it is believed that, the preferred range of the velocity differential between the perforating roll and the web is between about -20% (underspeed) and about +100% (overspeed). This range is dependent on many factors, such as, for example, physical properties of the web, operating tension, design of perforation (width of the land areas and spacing therebetween), interference between the blade(s) and the anvil(s), web speed, and other factors. This range cannot provide for all desirable variations of the spacing between the perforation lines and the product count without changing one perforating roll for another having a different number of the perforating surfaces.
Now it has been found that the problem may be successfully resolved by designing a novel differential-spacing perforating roll which allows much greater flexibility in controlling the length of individual sheets in the perforated web, while avoiding the perforation popping and vibration problems. Accordingly, an object of the present invention is to provide such a differential-spacing perforating roll. Another object of the present invention is to provide a process for making a perforated web using the differential-spacing perforating roll of the present invention.